Various polymers may be used in the manufacture of electronic devices, including, for instance, photoresists and organic-based dielectrics. Photoresists, for example, may be used throughout semiconductor device fabrication in photolithographic operations. A photoresist may be exposed to actinic radiation through a photomask. Where a positive-acting resist is used, exposure may cause a chemical reaction within the material resulting in a solubility increase in aqueous alkali, allowing it to be dissolved and rinsed away with developer. Where a negative-acting resist is used, cross-linking of the polymer may occur in the exposed regions while leaving unexposed regions unchanged. The unexposed regions may be subject to dissolution and rinsing by a suitable developer chemistry. Following development, a resist mask may be left behind. The design and geometry of the resist mask may depend upon the positive or negative tone of the resist; positive tone resist may match the design of the photomask, while a negative tone resist may provide a pattern that is opposite the photomask design. The use of photoresists may require several cleaning steps with a final clean of the mask before the next circuit design process step is implemented.
Organic-based dielectrics represent engineering polymers used to offer insulative properties to the microelectronic circuit. Examples of these chemistries include polyimide (PI) and poly-(p-phenylene-2,6-benzobisoxazole) (PBO) as manufactured by Hitachi-DuPont Microsystems. Another exemplary organic-based dielectric for electronic applications is bisbenzocyclobutene (BCB), manufactured by the USA-based, Dow Chemical Company. These polymers may be applied to the substrate in a similar fashion as photoresists using conventional spin, spray, or they may be slit-coated (which can be done, for instance in manufacturing FPDs). For these application reasons, organic-based dielectrics may often be referred to as spin-on dielectrics. Once the polymer is applied, the organic-based dielectrics may undergo a patterning process, but ultimately all of these systems lead to a final-stage cure, which may permanently fix the material in place by undergoing chemical and physical property changes. The final material may, for instance, exhibit both electrical and physical properties desirable for performance of the electric circuit. Once these organic-based dielectrics are fully cured, they are considered to be permanent, whereby, the need for rework would either require the use of aggressive materials such as strong acids or bases that likely would attack the substrate or adjacent metals or more practically, the rework condition would be considered as not commercially viable.
Positive photoresists may be based upon resins of the Novolac or polyhydroxystyrene (Phost) varieties chosen for high-resolution device processing in front-end semiconductor and flat panel display (FPD) manufacturing. Positive-tone systems represent the largest volume portion of photoresists produced globally and there are many suppliers. Exemplary suppliers of these systems for both semiconductor and FPD include, but are not limited to, the USA-based AZ Electronic Materials, the USA-based Rohm and Haas Company, and the Japanese company, Tokyo Ohka Kogyo Co. Ltd. In positive photoresist applications, a substrate may be etched by plasma processes, which may use gases of inert and chemical varieties to, for instance, produce both ionized and reactive species that travel through the mask and etch down into the substrate. During etching, ionized and reactive species may combine with atoms of the substrate, form a by-product, and that by-product is vented away via the reduced pressure of the plasma system. These same gaseous species may also impact the photoresist mask, for instance, by baking it into place and also ejecting carbon-containing by-products into the plasma. Photoresist by-products may mix with other species in the plasma and are continually directed down towards the substrate. These materials may condense to form a residue along the sidewalls of the etched features, producing a condition otherwise referred to as anisotropic etching, whereby species are highly controlled and directed into the substrate with little or no lateral loss. Upon completion, this etch residue may be removed along with the resist mask to prevent potentially deleterious effects on subsequent processes and lead to reduced device performance or device failure. Such residues and their associated resist masks, however, can be difficult to remove, normally involving the use of formulated stripper chemistries.
Negative photoresists may be chosen for more rigorous process conditions whereby more aggressive chemical or thermal exposure processes may be used. These negative photoresists include, but are not limited to, isoprene (rubber), acrylic, and epoxy-based resins. Cyclized isoprene (rubber) photoresists may be chosen for their high chemical resistance. Examples of these photoresists, for instance, may be obtained from Fujifilm Electronic Materials, Ltd. under the trade name SC-RESIST or HNR-RESIST. Negative-tone isoprene resin resists may be used in aluminum processing where a brief chemical etch may be used to remove metal surrounding the masked feature. Negative-tone acrylic photoresists may be chosen for wafer-level-packaging bump formation. Suppliers include, but are not limited to, the USA-based Printed Circuits Division of E. I. duPont de Nemours and Company under the trade name RISTON, and the Japan's JSR Corporation for dry-film and spin-on (wet) negative acrylics, respectively. Dry-film and spin-on acrylics may offer an ability to deposit thick layers from 25 microns (μm) to 120 microns (μm), used to pattern the corresponding solder bumps. Once the pattern is formed, metal deposition may occur by electroplating or screen-printing, a process that may expose the resist to heated acid or baking in excess of 250° C., respectively. Another exemplary negative resist is an epoxy system under the trade name of SU-8™, originally developed by International Business Machines (IBM) and now sold by the USA company, MicroChem Corporation, and Gersteltec Engineering Solutions, a Swiss-based company. The SU-8™ may be chosen for thick patterns that may exceed 300 microns, with a high-aspect ratio (i.e., height vs. width), and with the pattern definition to exhibit straight sidewalls. Because of the unique characteristics of the SU-8™ epoxy resin, photoresists of this variety may be chosen to manufacture large devices, and may include microeletromechanical systems (MEMS). The varieties of negative-tone photoresists may be different from positive, their cleaning (removal) practice may be even more rigorous. The SU-8™ photoresist may be considered to be a permanent system, removed only with more complex, time, and costly practices. Processes according to the present disclosure may be particularly advantageous for the removal of thick substrates like the SU-8™ photoresists.
As with many processes involving photolithography, it may be desirable to completely remove the photoresist from the substrate before proceeding to the next process. Incomplete stripping of the photoresist may result in irregularities during the next etching or deposition step, which may cause quality and yield problems. For example, during solder bumping, resist contamination may prevent metal solder from wetting to a metal pad during the board assembly reflow processes, resulting in yield loss in a finished assembly. The same photoresist contamination may be manifested as organic contamination in front end of line device (FEOL) patterning and may result in the same non-wetting problems in an etch or deposition process. Such irregularities, no matter how small, may continue to magnify the problem throughout manufacturing until, during final device assembly and testing, the condition may lead to poor mechanical and electrical contacts, which produce high resistance and heat, or worse, catastrophic electrical shorting.
Throughout each of these chemical processes, selectivity in cleanliness and high throughput should be met without failure. Any problems associated with a lack of performance, presence of residue, or worse, a rise in process complexity, each may result in reduced yield and increased cost.
The chemistry of positive-tone resists may be hydrophilic (polar) and amorphous (i.e., non thermoset and cross-linked), and it may be easier to clean (remove) using conventional solvents and/or chemical strippers. The resins for positive-tone chemistries may be based upon either Novolac (cresol, phenol-formaldehyde) or polyhydroxystyrene (Phost), with options of styrenated copolymer and/or acrylic/PMMA (polymethylmethacrylate). These chemistries may offer good adhesion and fixing to a wide variety of surfaces while the hydroxyl groups present in the various forms of Novolac (i.e. cresol, bis-phenol, etc.) may provide intermolecular hydrogen bonding that aids in aqueous solubility. This condition may combine during the photoconversion of the initiator diazonaphthoquinone (DNQ) in Novolac systems, while in Phost systems, the acid catalyzed de-protection of the ester forms the more soluble alcohol. When used during operating conditions up to and including 100° C., these systems remain soluble in polar solvents while their UV-exposure will produce counterparts that are soluble in aqueous-base.
The positive-tone resists may be used as primary imaging masks for plasma-based etching. During this process, species in the plasma may produce etch residue while exposing the mask to temperatures exceeding 150° C. Etch residue (e.g. side-wall polymer) may be comprised of by-products of the plasma with organic constituents of photoresist. The chemistry of the residue may comprise constituents of the substrate, metal topography, and plasma gases, to include silicon, gallium, arsenic, boron, phosphate, titantium, tantalum, tungsten, copper, nickel, aluminum, chromium, fluorine, chlorine, as well as carbon containing compounds. In Novolac systems that contain hydroxyl constituents, these elevated temperature exposure conditions may facilitate further reactions to form insoluble species. The reactivity of hydroxyl groups with halides and active metals, especially in the heated and acidic conditions of a plasma, to produce alkyl halides, esters, and, in some cases, high molecular weight polymers is known (Morrison, R. T. and Boyd, R. N., Organic Chemistry, 3rd Ed., Allyn & Bacon, Inc., Boston Mass., Ch. 16 (1973)). Cleaning of etch residue and overexposed photoresist masks resulting from the effects of hot plasma etching may require the use of chemical strippers processed at elevated temperatures for extended periods of time dependent upon the process and tool.
Measurements used to predict stripping challenges of bulk resins include, for instance, thermal analysis determination of glass transition (Tg). Relatively unchanged Tg values may be observed in positive-tone photoresists and similar amorphous systems (Fedynyshyn, T. et al., Proc. SPIE 6519, 65197-1 (2007)). Detectable increases of Tg in photoresists may be a function of the evaporative loss in solvent, which in turn, may depend upon the thickness of the photoresist coating. Most notable are observed increases in Tg with radiation and thermal exposure with polymer cross-linking (J. D. D'Amour et al., Proc. SPIE 5039, 966 (2003)). Such cross-linking of high temperature exposed Novolac resins and negative-tone systems is consistent with the presence of higher molecular weight species as detectable by increased values of Tg.
Cleaning (removal) of photoresist etch residue and the mask use complex chemical strippers composed of organic solvents, amines, water, reducing agents, chelating agents, corrosion inhibitors, and surfactants. The reducing agent, hydroxylamine, has been cited in the literature as a basic material that may facilitate dissolution of photoresist and its residue while offering protection of underlying aluminum metal features. The use of stripper chemistries may involve the delivery of large volumes of stripper to the substrate to be cleaned at a specific temperature for a given period of time.
As the industry continues to replace aluminum with copper to capture improved performance in their devices, the stripper chemistries must also be adjusted. Hydroxylamine may be acceptable for cleaning of aluminum devices; however, it may be too aggressive for copper. Device architecture using copper and low-K (dielectric constant, K), e.g. Cu/Low-K, may require fluorinated-based chemistries to remove silicon-laden etch residue. Amines and ammonia compounds are known to be complexing agents for Cu and may etch (attack) copper metal.
Negative photoresists used in forming wafer bumping metallization masks may include acrylic, styrenic, maleic anhydride or related monomers and copolymers. Such materials may be used to produce photosensitive thick films. These photoresists may be referred to as “acrylic” polymer systems due to the pendant groups on the main polymer chains, which include vinyl groups common to acrylics. The dry-film form of acrylic photoresists may be chosen where exposure to rigorous process conditions is required. As a result of this exposure, the cleaning of dry-film masks and residues may present a stripper challenge.
Resist stripping compositions that include aromatic quaternary ammonium hydroxide such as benzyltrimethylammonium hydroxide (BTMAH), a solvent such as an alkylsulfoxide, a glycol and a corrosion inhibitor and nonionic surfactant may not completely remove many dry-film resists from a wafer surface. Similarly, compositions using pyrrolidone-based solvents such as N-methylpyrrolidone (NMP) exhibit the same drawback in that they cannot achieve complete removal of many dry-film resists. In general, compositions that include a quaternary ammonium hydroxide such as tetramethylammonium hydroxide (TMAH) in NMP may not completely dissolve many dry-film resist. As discussed above, incomplete dissolution may produce particles that can become a source of contamination resulting in yield loss.
Similar experience may be noted for negative-tone photoresist of the rubber-based resin variety. Stripper chemistries that may be used to clean residue and masks resulting from rubber photoresists may include a hydrocarbon solvent and an acid, commonly a sulfonic acid. High acidity may be required for performance and emulsification of hydrolyzed rubber components. Representative inhibitors include, but are not limited to, mercaptobenzotriazole (MBT) and related triazoles to, for instance, prohibit attack upon adjacent metallic features. An exemplary inhibitor for these chemistries includes catachol, a toxic and carcinogenic material. Further, rinse steps for hydrocarbon strippers of this variety should use isopropanol (IPA) or related neutral and compatible solvents. This rinse practice, albeit a cost increase, may reduce the effects of metal attack to adjacent metals due to a pH-drop during water mixing with constituents of the stripper. Due to compatibility issues, wastes from the use of hydrocarbon-based strippers should be segregated from normal organic streams in a microelectronic fabrication.
Further, a cleaning tool may provide control in the process. Variability between part batches may be reduced by the operation of the tool. Barring any mixing or chemical adjustments made by the unit, the variables available to the tool for control include temperature, agitation, and time. With an ever-present intensive pressure to increase throughput in a manufacturing line, a constant emphasis is to decrease the process time. Again, without a change in chemistry, temperature and/or agitation may be increased with the expectation that polymer dissolution rates may increase resulting in shorter process time. However, other reactions that are contradictory to the objectives of the process, such as corrosion rate, may also increase with increased temperature and/or agitation. Continued loading of the stripper chemistry with the organic substance may cause a reduction in bath life and may accelerate the observation of residue or other phenomena that indicate a drop in performance. Further all wafers do not experience the exact same stripping environment, thus causing some amount of process variation.
On the temperature continuum, bath life may be facilitated by increasing temperature and/or agitation. Where agitation should be controlled to protect substrate features, bath life conditions may be increased through increased polymer dissolution with increasing temperature. There is a fundamental safety limit as communicated by industry guidelines (SEMI S3-91, Safety Guidelines for Heated Chemical Baths). Particularly when processing in baths, in accordance with SEMI, liquid over temperature shall be controlled at not more than 10° C. above the normal operating temperature of the liquid, where the typical operating temperature does not exceed the flashpoint of the liquid. Many companies set policy that is more restrictive such as operating at 10° C. below the flashpoint and setting the over temperature to be the flashpoint. These criteria and others may be observed in the processing of flat panel displays (FPDs).
Resist stripping at a FPD manufacturing plant may occur on large substrates traveling on a conveyor from one chamber to another. The resist may be stripped from the panel by a stripper delivered by a sprayer that floods the entire glass surface, traveling to a rinse stage where distilled, deionized, or demineralized water or an alternative solvent may be sprayed onto the surface, and the process may be completed with a drying step that may include a hot air knife. Stripping may be supported by at least two product tanks that are separate and distinct and arranged in-line with the flow direction of the parts. Substrates entering the tool may be first “washed” by the chemistry in the first tank. The stripper may be sprayed onto the substrate surface, and upon reacting with the resist and flowing off of the substrate, it may be collected and returned to the tank where it may then be heated and filtered such that any suspended and undissolved materials may be removed from the bulk chemistry. The filtered and heated stripper may be then cycled back to the spray chamber where it may be delivered to the substrate in a continuous manner that optimizes the resist stripping process.
As the part travels on the conveyor from the first chamber supported by tank #1 to the next chamber supported by tank #2, there may be a purity change in the stripper. Although the conditions of operation for tank #2 may be the same as that for tank #1, the amount of resist present may be lower than that for tank #1. Typical processing times may be defined for chamber #1 to offer a dwell time of the chemistry in contact with the resist that may optimize resist stripping and maximum removal. Over time, tank #1 may reach a maximum loading capacity for dissolved resist and a decision to replace the contents may be necessary. When this occurs, the contents of tank #1 may be sent to waste and replaced by the contents of tank #2. The contents of tank #2 may be replaced with fresh stripper (i.e. pure stripper). In this manner, the system may be said to operate in a counter-current fashion. Namely, the process flow of parts may be “counter” or opposite to the flow direction of the chemistry. By using this practice, tanks #1 and #2 may become the dirty and clean tanks, respectively. In other words, the unwanted resist may be concentrated in the front of the line while the cleanest chemistries remain near the end whereby after this point, the product substrate may be rinsed and dried.
The configuration given above for the FPD example may be consistent with many, if not all, in-line bench style tools and with many batch style-processing tools. In a bench tool, parts may move from one station to another while the tanks are at fixed locations. In a batch style tool, the parts may rotate but remain at a fixed location, while the chemistry may be delivered by spraying. There may be two tanks, the tool may pump from one or the other and carry-out counter-current cleaning designs by the use of “dirty” and “clean” tanks.
There is a yet unsatisfied need to achieve selectivity during processing with these formulated strippers. Namely, as the use of more aggressive chemistries may be put into practice to achieve a desired cleaning performance in ever reducing time, this practice should be met without damage to sensitive metals and the underlying substrate. This may be challenging as many of the acids or alkalis of choice may rapidly “spike” the pH of the system, once they are mixed with water during the rinse step, causing galvanic corrosion to substrate metals. During the rinse stage on a FPD line, water may be sprayed on the heated glass surface that contains residual stripper. No surfactants are used in a FPD line, for fear that a foam condition may occur and cause catastrophic failing of filters, pumping of dry air bubbles, and worse, contaminating the fabrication facilities by overflowing stripper that may trigger electrical shorting and lead to a fire. Since no surfactants are used, there may be irregular diffusion due to rising surface tension from the organic stripper to the aqueous condition. Irregular mixing and spreading may cause momentary dead spots on the panel, which may contribute to accelerated corrosion. The corrosive by-product and foaming condition may be avoided through rinsing with neutral solvents such as isopropanol (IPA). Although this practice may be acceptable to several FPD manufacturers, it is expensive and a flammability hazard.
There is a need, accordingly, for new processes, apparatus and improved stripping compositions that may remove the processed resist in a rapid manner while maintaining safety towards the underlying metallurgy during rinsing with distilled, deionized, or demineralized water, and preventing corroding, gouging, dissolving, dulling, or otherwise marring the surfaces throughout the entire process. Further, growing initiatives exist within the industry to move toward being “green.” A green process and the associated chemistries are those which may reduce or eliminate the use and generation of hazardous substances. According to the American Chemical Society's Green Chemistry Institute, there are twelve (12) principles that help to define a green chemistry.
Where organic dielectrics are used, there may be a continuing need for processes and compositions that may be used to effectively re-work a cured polymer by dissolving and cleaning the unwanted material from the underlying substrate. In cases of positive photoresists, there may be a similar and continuing need for processes, apparatus, and compositions to effectively remove polymer from a substrate without deleterious effects to adjacent metal features. Finally, in the case of negative-tone photoresists, the same need exists for processes and compositions to effectively remove polymer from a substrate without deleterious effects to adjacent metal features.
While there is a desire to address the removal needs of organic substances with unique compositions, there also, is a challenge to design a process that is supported by an apparatus that may enable rapid processing of parts, that may enable a higher level of consistency of the chemistry applied to the parts, and rinsing without deleterious effects to the substrate. There is a continuing emphasis for the microelectronics industry to be green through improving the safety of operations, reducing the use of chemistry, and reducing the generation of hazardous waste. Taking these challenges together, there is a need to provide a consistent process, which uses compositions of matter that vary depending upon the end product to be produced and the performance needs of the polymer or residue to be removed, which provides high performance, high throughput, a green process, at a reduced cost of ownership.